Halogenated organic compounds, particularly aliphatic organics such as the chloroethanes and chloroethenes, especially tetrachloroethene (also known as perchloroethene or PCE), trichloroethene (TCE), dichloroethene (DCE) and vinyl chloride (VC), are common groundwater contaminants in the United States and elsewhere. PCE is a solvent used to clean machinery, electronic parts, and clothing. Many halogenated organics and their degradation products are suspected carcinogens. The degradation products can often be degraded by indigenous aerobic microorganisms, but, PCE and other recalcitrant halogenated organics, are degraded anaerobically. Enhancing or modifying subsurface conditions can provide anaerobic conditions that promote reductive dehalogenation. PCE impacted groundwater, for example, has historically been treated using conventional pump and treat or air sparging methods. These approaches require large capital, operation, and maintenance investments and still take a long time to remediate the groundwater. Therefore, efforts are underway to develop biological methods for remediating the more recalcitrant halogenated organic compounds typified by PCE.
PCE and other recalcitrant compounds can be degraded by anaerobic microorganisms under reducing conditions. In the laboratory, anaerobic PCE degradation can be enhanced by adding trace nutrients, and an electron donor substrate such as lactate, under reducing conditions. Although the chemistry has been conceptually demonstrated in the laboratory, the laboratory results are not necessarily applicable to in situ remediation because of numerous differences from the controlled laboratory environment. In particular, aerobic bacteria, not present in the laboratory models, can utilize the electron donor as an energy source and can grow to high numbers when the substrate is added. Moreover, the aerobic bacteria, which have little if any capacity to degrade the recalcitrant halogenated organic compounds, deplete the added substrate and require that additional substrate be added to accomplish the original purpose of degrading the halogenated organic compounds.
In prior field methods, an anaerobic environment is created over a long time frame, by allowing aerobic bacteria to use the added excess electron donor, typically lactate, as an energy source, thereby depleting oxygen from the groundwater and the saturated matrix, which can include soil or rock. Once the level of dissolved oxygen in the groundwater reaches approximately 1 mg/L, indigenous anaerobic bacteria can begin to degrade the PCE.
However, the prior field method is disfavored because once the dissolved oxygen reaches a suitably low level, and before any PCE is degraded, a substantial amount of the added lactate has been utilized by the aerobic bacteria, thereby requiring input of additional electron donor. Existing biological methods are further hampered by the fact that the aerobic growth that reduces the dissolved oxygen concentration has the undesirable side effect of clogging the subsurface, soil matrix or injection well. Accordingly, it would be desirable to devise a method that facilitates in situ anaerobic dehalogenation while avoiding an aerobic growth stage.